1. Field of the Invention
The present invention generally relates to an electrolysis process of an aqueous alkali metal halide solution using an asbestos diaphragm or an ion exchange membrane, more specifically, to a process for preventing a low hydrogen overvoltage cathode from degrading in activity at the time of shutdown of an electrolytic cell for use in the foregoing electrolysis.
2. Description of Prior Art
As electrolysis processes of an aqueous alkali metal chloride solution, a mercury electrolysis process and an asbestos diaphragm process have been practiced on an industrial scale. However, the former is being switched to the latter because of environmental pollution. The asbestos diaphragm process, notwithstanding, has numerous disadvantages including low quality product and great consumption of energy, so that an ion exchange membrane electrolysis process has been developed. It is surmised that in the near future alkali hydroxide will be produced by the asbestos diaphragm electrolysis process and the ion exchange membrane electrolysis process in Japan.
The asbestos diaphragm electrolytic cell and the ion exchange membrane electrolytic cell involve two different systems of monopolar type and bipolar type, and mild steel has been heretofore served as a cathode in every type. Hydrogen overvoltage of mild steel, nontheless, is as high as 0.3 to 0.4 Volt and thus the study on low hydrogen overvoltage cathodes to save energy cost is being actively made. For example, a variety of processes including plating or spraying of nickel or a nickel alloy are proposed by Japanese Patent Non-examined Publication Nos. 112785/79, 63686/82, 82483/82, 114678/82 and the like.
However, it has been pointed out that when a low hydrogen overvoltage cathode prepared by those techniques is installed to the asbestos diaphragm or the ion exchange membrane electrolytic cell, activity of the cathode deteriorates to thus result in an increase in hydrogen overvoltage. The phenomenon appears notoriously when the operation of the specified electrolytic cell under operation was shut down for reasons of inspection, changing of the asbestos diaphragm or ion exchange membrane, exchange of electrodes and so on.
As a rule, the operation of the specified electrolytic cell among a plurality of electrolytic cells under operation is shut down by the use of a short-circuit switch. In such a case, to the electrolytic cell shut down an electric current reverse to the original electrolytic current begins to flow instantaneously when short-circuited. Then, a cathode becomes to be an anode and dissolution of metal occurs. The dissolution of the metal presumably occurs selectively from high active portions and the activity before shutdown is no longer expected even when the operation is resumed. As the result, cell voltage increases.